Load compensating temperature control system



Jan, 27, 1942.

LOAD COMPENSATI'NG TEMPERATURE CONTROL SYSTEM L. L. CUNNINGHAM OriginalFiled May 27, 1933 auf dustrial heating systems.

Patented Jan. 27, 1942 *UNIT-ED `s'rA'niS PATENT OFFICE LoancoMPENsATlNG TEMPERATURE r coNTaor. SYSTEM Lewis L. Cunningham,Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company,Minneapolis, Minn., a corporation of Delaware Original application May27, 1933, Serial No.

673,236. Divided and 1935, SerialluNo. 32,221

3s claims.

The present invention relates to temperature control systems in generaland particularly to in- Inl the practical control of a heating system,for instance, it hassometimes been found that the positioning of a heat-control instrumentality in accordance with the temperature and/orpressure condition to be controlled permits of undesirably largelfluctuations from'normal of the temperature and/or pressure conditionif the demand or this application July 19,

loadA on the heating system varies ln a sub's'tim-l tial amount. Underthese conditions, it is desirable primarily to position the `heatcontrol instrumentality in accordance with the position 'oi a movablemember .responding to the temperature and/or pressure condition and alsoto change the positionof the heat control instrumentality as a result inchanges in load, such as positioning it ing of a control instrumentalityprimarily in accordance with the position of a movable member and alsoin accordance with the deviation of that, or another, movable memberfrom its normal desired position and/or in accordance with the length oftime that such abnormal deviation persists and, in addition, to adjustthe position of the control instrumentality upon initial reversemovement oi'. the movable member.

More specifically, the invention has -for an object, the provision of asystem in which a normally balanced electrically controlled means isunbalanced by the movement of a movableimem- 'ber from its normaldesired position and is additionally unbalanced in proportion to theamount .of deviation of the movable member from its normal desiredposition and/orin accordance in accordance with the deviation of themovable f 'member from its normal position and/or in acmovable memberupon initial reverse movementl of the movable member. In other words.upon initial reverse movement of the movable member. the heat controlinstrumentality is given an initial with the length of time suchdeviation persists and/or as a result oi.' initial reverse movements oi'the movable member. The system is such that the unbalancing of thenormally balanced elec- ,trically controlled means or balanced solenoidoperates a switching mechanism in control of an electrically controlleddevice which positions a control instrumentality in accordance with thedegree of unbalancing of the electrically controlled means Vand alsoelectrically rebalances said means.

movement in a direction to check or counteract the change which causedsuch reverse movement of the movable member. This initial change inFurther objects of the invention will be found inthe drawingillustrating the same, the detailed description and the claims.

For a. more complete understanding of the invention, reference may behad to the following detailed description and the single accompanyingdrawing which is a diagrammatic showing 0f one form of my invention.

This appication is a division of my copending application Serial No.673,236, tiled May 27, 1933 now Patent No. 2,160,400, dated May 3o, 1939posltionof the heat'control instrumentality is in l addition to theusual movements resulting from any movement of the movable member andmay also be in addition to the corrective movements i applied as aresult" of a deviation of the movabley member from its desired normalposition in accordance with the amount of such deviation and/or inaccordance with the length of time such deviation persists. y

An additional object of the invention is the positioning of a controlinstrumentality primarily in accordance with the position of a movablemember and also as a result oi' initial reverse movements of the movablemember.

Another object of the invention is the positionv.entitled Remote controlsystems." y v Referring to the single drawing, a temperature vcontrolling device is herein shown in the form of a valve III which hasits stem Il connected to a rack l2. The rack i2 is engaged by a pinioni3 which is in turn secured to the main shaft I4 oi a motor mechanismgenerally indicated at I5. This main shaft il is connected to a suitablereversible motor means through reduction gearing indicated at Ii. Thereversible motor means is herein shown as comprising a pair of rotors l1and Il that are secured to a common rotor shaft I9, eld windings 20 and2i being associated with the respective rotors il and i8.' It will beunderstood that any other suitable type of reyersible motor means couldbe utilized.

The field windings 28 and 2| are primarily controlled by a balancedrelay which comprises a pair of relay windings 22 and 23. These mayeither be separate windings or a center-tapped winding. Thesetwo-windings 22 and 23 control a single plunger or armature 24 which isconnected to a relay switch arm 25 througha nonmagnetic resilientconnection, such as the spring 28. The switch arm 25 cooperates with apair of spaced contacts 21 and 28 between which it is disposed.

The field windings 28 and 2| are additionally controlled by a pair oflimit switches, the first comprising switch arms 38 and 3| and thesecond comprising switch arms 32 and 33.

Upon movement of relay switch arm 25 into engagement with contact 21,field winding 2| is energized as follows: line wire 34, wire 35, relayswitch arm 25, contact 21, wire 38, limit switch arms 33 and 32, wire31, field winding 2| and wire 38 to line wire 38. Similarly, engagementof relay switch arm 25 with contact 28 causes energization of fieldwinding 28 by a circuit as follows: line wire 34, wire 35, relay switcharm 25, contact 28, wire 48, limit switch arms 38 and 3|, wire 4|, fieldwinding -28 and wire 38 to line Wire 39.

The respective energizations of relay windings 22 and 23 are controlledby a balanced solenoid mechanism which includes a pair of solenoid coils45 and 48 that may well be formed by providing a single solenoid coilwith a center-tap. 'I'hese solenoid coils 45 and 48 control a singleplunger or armature 41 which is connected to a switch arm 48 through anon-magnetic connection, such as the spring 48. The switch arm 48cooperates with a pair of/spaced contacts 58 and 5| and is disposedbetween the same.

The respective energizations of solenoid coils 45 and 48 are controlled,in part, by a controlling potentiometer which includes a controlresistance 58 and a cooperating control contact arm or movable member58. This movable member 58 may be controlled in any manner, eitherautomatically or manually, and is herein shown as controlled by atemperature responsive element in the form of a bimetallic coil 51.

The energizations of solenoid coils 45 and 48 are further controlled bya balancing potentiometer which comprises a balancing resistance 58 anda cooperating balancing contact arm 58 which additionally engages acontact plate 88.

This balancing potentiometer and the two heretofore described limitswitches are all controlled by a mechanism which will now be described.This mechanism includes a piece of insulating material 8| or actuator towhich the balancing contact arm 58 is secured. 'I'his piece ofinsulating material 8| is provided with an outwardly extending finger 82which is adapted to engage the switch arm 38 and move the same fromengagement with the switch arm 3|. The piece of insulating material 8|is further provided with an extension or finger 83, a portion of whichis adapted to engage the limit switch arm 33 and move the same fromengagement with limit switch arm 32, there being a lever 84 piv-U otallyconnected to the end of finger 83 as indicated at 85. This lever 84 ispivoted to a suitable support at 88 and supports an adjustable abutmentindicated at 81. A second lever 88, which is pivoted at 88, is arrangedto engage the abutment 81. This lever 88 supports a roller 18 whichengages a cam 1|- that is secured to the main shaft I4 of the motormechanism I5. A'

coiled spring 12 biases the` actuator 8| to the right so that theabutment 81 is held against the lever 88 and the roller 18 in turn isheid against the cam 1|. It will be evident that upon clockwise movementof main shaft I4, as viewed from the left, the spring 12 will pull theactuator 8| towards the right. Similarly, upon counterclockwise rotationof the main shaft |4, as viewed from the left, the cam 1| will move theactuator 8| towards the left against the action of spring 12. Ifdesired, this motor mechanism |5 may operate a second potentiometerwhich may be utilized as the controlling potentiometer for anothersimilar motor mechanism. This second potentiometer comprises acontrolling resistance 15, a controlling contact arm 18, and a contactplate 11.

Low voltage power is supplied to the solenoid coils 45 and 48 and to therelay windings 22 and 23, as well as to the associated controlling andbalancing potentiometers, by a step-down transformer 88 having a highvoltage primary 8|, that is connected to the line wires 34 and 38, and alow voltagesecondary 82. The solenoid coils and 48, in series, areconnected across this secondary 82 by means of wires 83, 84, 85 and 88.Similarly, the relay windings 22 and 23, in series, are connected acrossthe secondary 82 by means of wires 83, 81, 88 and 88. The junction ofrelay windings 22 and 23 is connected to the switch arm 48, through aprotective resistance 88, by means of a wire 88.A The contact 58 isconnected to a small number of turns of the solenoid coil 45 by means ofa wire 8| and the contact 5| is connected to a small number of turns ofthe solenoid coil 48 by a wire 82. The control resistance is connectedin parallel with the sries connected solenoid coils 45 and 48, throughresistances 93 and 84, by wires 85, 88, 81, 98, 88

` and |88. The wires 88 andy |88, instead of being directly connected tothe respective resistances 83 and 84, are connected to contact arms |8|and |82 which respectively engage the resistances 83 and 94 for apurpose that will be hereinafter set forth. The balancing resistance '58is connected in parallel with the controlling resistance 55 by means ofwires |83 and |84. The junction of solenoid coils 45 and 48 is connectedto the bimetallic element 51, and therefore to the control contact arm58, Athrough a manually operable rheostat |85 by means of wires |88 and|81. Similarly, the junction of solenoid coils 45 and 48 is connected tothe contact plate 88, through a rheostat |88, by means of wires |88 and|88.

Assuming that the valve I8 is in the fuel line of a furnace and thatbimetallic element 51 responds to the furnace temperature, the valve I8will be moved to positions corresponding to the temperature ofbimetallic element 51 in the manner set forth in my copendingapplication Serial No. 673,236 of which this application is a division.With the parts in the position shown, the valve I8 is substantially halfopen and the furnace temperature is at the desired point. If the furnaceis operating under a small load, the temperature thereof will rise withthe result that bimetallic element 51 will move movable member 58 alongcontrol resistance 55 toward the left hand end thereof which action willpartially close the valve I8 controlled by shaft I4 until a balance isreached wherein the heat input maintains the furnace at an eventemperature. This temperature, however, is somewhat higher than thatdesired as evidenced by movement of movable member 58 to the left ofits'center position. The furnace temperature can be maintained fairlyclose to the desired point by making the over-all length of controlresistance 55 correspond to a relatively small temperature differentialin the furnace. However, if the over-all length of control resistance 55is made to correspond to a very quired in commercial processes, theremay be a resultant hunting of the valve l and the system may no longerproperly modulate or proportion the position of the valve I0. 'As aresult, when itl is desired to control the furnace temperaturedifferential within very narrow limits under varying load conditions, itis desirable to provideadditional mechanism whereby'the position oi' thevalve l0 and shaft I4 may be adjusted auto-- matically in accordancewith the departure of the furnace temperature Afrom normal and/or inaccordance with the length of. time that such a departure persistsand/or upon initial reverse changes in furnace temperature. Theadditional mechanism which will now be described makes this automaticadjustment possible.

-The contact ngers |0| and |02 are secured to shaft ||5. The shaft ||5carries a ratchet wheel ||6 and a pinion 1. Pinion H1 meshes with anidler gear 8 which in turn meshes with a second pinion ||9 mounted upona shaft |20. Shaft carries a second ratchet wheel 2| gize the fieldwinding |24 of a timing motor generally indicated at |25, wires |26 and|21 connecting line wires |22 and |23 to the opposite ends of fieldwinding |24. The rotor |28 of the timing motor drives a reductiongearing herein shown as a worm |29 and a worm wheel |30. A crank pin |3|is carried by worm wheel v| 30 and. the free end of this crank pin |3|extends into Va. sot |32 which is formed in a lever |33, one end ofwhich is pivoted as shown at |34. A vertical actuator has one of itsends pivoted to the free end of the lever |33 as shown at |36. Theactuator |35 is constantly reciprocated vertically by the timing motor|25 and associated mechanism at the rate of about once every two minutesand when the actuator |35 is positioned substantially vertically, itreciprocates freely without engaging either of the ratchet wheels If thelower free end of actuotor |35 is moved to the left, it engages theteethyof ratchet wheel ||6 upon upward movement so as to rotate shaft||5 and shaft |20 through pinions ||1 and ||9 and idler gear ||8, in acounterclockwise direction as viewed from the right. Likewise, if thelower end of actuator |35 is moved to the right, it engages the teeth ofratchet wheel |2| during its upward movements and rotates shafts |20 and||5 in aclockwise direction.

The position of actuator |35 is controlled by a plunger to which it isconnected by a link |4| and a spring |42. The plunger |40 is controlledby a pair of solenoid coils |43 and |44 which have adjacent endsinterconnected and their free ends connected to the secondary |45 of atransformer by wires |46 and |41. The primary |48 of the transformer isconstantly energized by the line wires |22 and |23.; A compensatorcontrol potentiometer comprises a resistor |50 and a movable member |5|.The movable member |5| is shown as connected to bimetallic element 51 bya connection |52. One

end of the corrector resistance l|50 is connected to wire |46 andsolenoid winding |43, through Line Wires |22 and |23 constantly ener--point indicated at .of plate |19 comprises any well known type of |82is connected to contact by means of wires, |54 of corrector resistance aprotective resistance' |58, and |55. The other-end/ |50 is connectedtowire |41 and solenoid winding |44, through a protective resistance|56,by wires |51 and |58. Movable member |5| is connected by a wire 7|59to a manualy operable contact finger |60 which may be moved acrossaresistance |6|, one end of which is connectedto the junction ofsolenoid windings |48 and `|44 by a wire |62. Contact finger |60 andresistance |6| comprise the sensitivity control for the correctivemechanism and itsfunction will be described under the vheading Operation'I'he shaft |20.V may control' a corrector1 indi' cating potentiometercomprising a movable contact ng'er |65' and resistance |66 which controlsolenoid windings |61 and |68 `that in tum position an indicating needle|69. The solenoid windings |61 and |68 and the. corrector-indicatingpotentiometer are'v connected together through suitable protectiveresistances and are also connected to the secondary |45 in the mannerpreviously set rfortlrin respect to similar arrangements.

'Ihe movable member 56 not only engages control resistance 55 but alsoextends therebelow and carries a pair of contacts |15 and |16 which areadapted, upon reverse movements of movable member 56, to .respectivelyengage contacts Contacts |11 and |16 are carried by a plate |19 Awhichis herein shown, for convenience of illustration, as being pivoted at a|80. The pivotal mounting frictional arrangement whereby the plate |19remains in any position to -which it is moved. In actual practise, theplate |18 is pivoted concentrically with the center of movement of themovable member 56 in order that contacts |15, (|16, |11, and |18 willretain their proper alignment.

i* first rheostat comprises a resistance 8| and a manually adjustablecontact finger |82. One end of resistance |8I is connected to thejunction of wires and-96. The contact finger |18 by a wire |63. Asimilar rheostat comprises a resistance |84 and manually adjustablecontact finger |85. The resistance |84 is connected to the junction ofwires 98 and 99. The contact finger' |85 is connected to contact |11 bya wire |86.

Operation open position, the temperature of the furnacei's at thedesired point with the result that movable member 56 is contacting thecentral'portion of control resistance 55, contacts |15 and |16 are outof engagement with contacts |11 and |18 respectively and the movablemember |5| is contacting the center portion of corrector resistance |50.Fieldf windings 20 and 2| are therefore both de-energized and theactuator |35 is in a vertical position wherein it is reciprocated bytiming motor |25 without engaging either of the ratchet Wheels ||6 and|2|.

If the temperature of the furnace should rise, bimetallic element 51will move movable member 56 along control resistance 55 bringing contact|16 into engagement with contact 16. Engagement of contacts |16 and |18places the effective portion of"resistance 8I in parallel with the lefthand portion of control resistance 55, thereby decreasing the voltagedrop across solenoid coil 45 a predetermined amount which is dependentupon the adjustment of contact finger |82. The changed position ofmovable member 56 on control resistance 55 likewise lowers the voltagedrop across solenoid coil 45. The voltage drop across solenoid coil 46is increased and the resulting unequal energization of solenoid coils 45and 46 causes plunger 41 to move to the right with the result thatswitch arm 48 engages contact 5| to place the small number of turns ofsolenoid coil 46 and the resistance 89 in parallel with relay winding 22whereupon plunger 24 moves to the right and energizes field winding 20.Shaft I4 thereupon is rotated clock-wise as viewed from the left. Suchrotation of shaft I4 begins to close the valve l controlled thereby andbegins to move balancing contact arm 59 to the right along balancingresistance 58 and contact plate 60. Balancing contact arm 59 must notonly move along balancing resistance 58 enough to balance the effect ofthe new position of movable member 56 on control resistance 55 but mustalso move an extra amount sufficient to overcome the unbalancingresulting from the engagement of contacts |16 and |18. In this manner,the initial movement of movable member 56 to the left results in aninitial relatively large adjustment of the valve l0 in addition to thatadjustment resulting from the movement of the movable member to a newposition on its control resistance.

'I'his movement of bmetallic element 51 has also moved movable member tothe left along resistance |50 whereupon the voltage drop across solenoidwinding |44 is made greater than the voltage drop across solenoidwinding |43 and plunger |40 moves to the right. This action moves thelower free end of actuator |35 to the right and when actuator |35 isnext raised by timing motor and the associated mechanism, ratchet wheel|2| is moved in a clockwise direction as viewed from the right. Shaft||5 is therefore likewise rotated in a clockwise direction with theresult that the resistance of the circuit comprising wire 81, resistance33, contact finger |0|, and wire 96 is decreased and the resistance ofthe circuit comprising wire 99, resistance 84, contact finger |02, andwire |00 is increased. This change in resistances of these circuitsagain unequalizes the voltage drops across solenoid coils 45 and 46whereby solenoid coil 46 is again energized to a greater degree thansolenoid coil 45. Plunger 41 therefore again moves to the right and thisresults in a further energization of field winding 20 whereupon shaft |4again rotates in clockwise direction las viewed from the left. 'Thisaction closes the valve |0 to a greater degree and likewise movesbalancing contact arm 58 closer to the right hand end of balancingresistance 58 to again rebalance the voltage drops across solenoid coils45 and 46 whereupon field winding 20 is again deenergized.

The shaft ||5 will be intermittently rotated in a clockwise directioneach time the actuator |35 is raised until the furnace temperature hasreturned to normal and movable member 5| is brought back to the positionshown in the drawing. 'I'he angular rotation of shaft ||5 each timeactuator is raised depends upon the amount of displacement of the lowerfree 'end of actuator |35 away from vertical and toward the right whichin turn is dependent upon the deviation of the furnace temperature fromnor'- mal as indicated by the position of movable member |5|L Thecorrective movements of the valve I0 toward closed position aretherefore made in accordance with the departure of the furnacetemperature from normal, and in accordance with the length of time suchdeparture persists. The time factor is by virtue of the timing motor |25which reciprocates actuator |35 preferably at the rate of a complete upand down movement every two minutes.

Continued rise in furnace temperature resulting in further movement ofmovable member 56 along control resistance 55 rotates plate |19 aboutits pivot |80. Such further movement also again unbalances the voltagedrops across solenoid coils 45 and 46 resulting in further closing ofthe valve |0. Continued rise in furnace temperature also moves movablemember |5| further along corrector resistance |50 whereby the voltagedrop across solenoid coil |44 is further increased and the voltage dropacross solenoid coil |43 is further decreased. Plunger therefore movesfurther towards the right and each subsequent upward movement ofactuator |35 will rotate shaft ||5 in a clockwise di rection a largeramount than formerly, whereby increased amounts of correction will beapplied to the system.

'Ihe various closing movements thus supplied to the valve |0 will checkthe rise in furnace temperature but as long as the furnace temperatureremains away from the normal desired value, further corrective movementswill be applied. Finally the furnace temperature will begin to lower.Initial lowering of the furnace temperature will cause movable member 56to disengage contact |16 from contact |18 thereby removing theresistance |8| from its parallel relationship with the left-hand portionof the control resistance 55. The voltage drop across solenoid coil 45will thereupon be increased and the voltage drop across solenoid coil 46will be similarly decreased. As a result, switch arm 48 engages Contact50 whereupon the small number of turns of solenoid coil 45 and theresistance 88 are placed in parallel with relay winding 23 and plunger24 moves to the left and brings relay switch arm 25 into engagement withcontact 21 thereby energizing field winding 2|. Shaft I4 is thereuponrotated in counter-clockwise direction, as viewed from the left, andvalve I0 is moved in an opening direction. Balancing contact arm 58 ismoved along balancing resistance 58 toward the left hand end thereof torebalance the voltage drops across solenoid coils 45 and 46. In thismanner, separation of contacts |16 and |18 causes an initial smallopening movement of valve I0. Also, part of this movement is the resultof movement of movable member 56 along control resistance 55.

A slight further movement of control member 56 to the right in responseto a furthe!` drop in furnace temperature will cause contact |15 toengage contact |11, whereby resistance |84 is placed in parallel withthe right hand portion of control resistance which again increases thevoltage drop across solenoid coil 45 and de creases the voltage dropacross solenoid coil 46, resulting in a further opening of the valve I0.In this manner, a small fall in furnace temperature results in aninitial relatively large opening movement of the valve.

If the furnace temperature continues to fall until the desired normal isreached, the voltage drops across solenoid coils |43 and |44 will berebalanced and the actuator |35 returned to its vertical positionwherein it does not engage either of the ratchet wheels ||6 or |2| andfurther corrective movements due to a departure from normal will nolonger be applied.

The movement of control member 56 to the right along control resistance55 also increases the voltage drop across solenoid coil 45 and decreasesthe voltage drop across solenoid coil 46 to open the valve an amountcorresponding to the movement of the movable member 56.

I! the furnace temperature falls below the desired normal, movablemember will be moved to the right of the center of corrector resistance|50 resulting in a greater voltage drop across solenoid coil |43 thanacross solenoid coil |44 and actuator |35 will be moved to the left ofits vertical position. Actuator |35 will thereafter engage the teeth ofratchet wheel ||6 upon each of its upward movements and shaft ,H5 willthereupon be rotated in a counterclockwise direction, when looking fromthe right, to intermittently increase the effective amount of resistance84 and decrease the effective amount ofresistance 98. 'I'hus the valveI0 is given intermittent opening movements in an effort to restore thefurnace temperature to normal as long as it remains below normal.

Whenever the furnace temperature becomes normal so that movable member56 again contacts the center of control resistance 55 and the movablemember |5| contacts the center of corrector resistance |50, all furthercorrective movements of the valve I0` will cease. However, the valve isnot necessarily in its mid position since it may be displaced therefromin either direction, depending upon the position which contact iingers|0| and |02 are left in respect to their 'respective resistances 93 and84 and also depending on whether or not either of contacts or |16 is inengagement with its respective contact |11 or |18. Such new position ofthe valve should exactly balance the new load condition imposed .on thefurnace. From the foregoing description of the operation of theapparatus of this invention, it will be apparent that this inventionprovides for the positioning of an instrumentality to be controlled inAaccordance with four diilerent factors. First,

the instrumentality is primarilypositioned in accordance with themovementsof a movable member. Second, the instrumentality is given aquick initial movement whenever the movement of the movable memberreverses. Third, the instrumentality is positioned in accordance withthe degree of departure of the movable member from its desired normalposition. And fourth, the instrumentality is positioned in accordancewith the length of time such departure persists.

As previously stated, in this specific embodiment of the invention, aspeed of two minutes has been found suitable for a complete revolutionof crank pin |3| but it will be appreciated that other speeds may bemore desirable under varying conditions of operation and when theapparatus is applied to different processes.

In the description of the operation of this system, the various movementof the valve I0 as the result of movement of the movable members 56 and|5| have been segregated and separately explained for reasons ofclarifying theexplanation, but in actual operation these movements willall overlap and occur simultaneously to a great extent.

This system includes several adjustments by reason of which theapparatus may be easily adapted to control any one of a number ofdifferent devices. If the rheostat |05 is adjusted to include less ofits resistance in the circuit, then a smaller movement of movable member56 along control resistance 55`will be required to unbalance the voltagedrops across solenoid coils 45 and 48 suiilclently to move the switcharm 48 into engagement with its associated contacts 50 and 5|. In thismanner, a smaller temperature change is necessary to obtain a givenmovement of the` instrumentality lto be controlled. Adjustment of therheostat |05 in a direction to include more of its resistance in thecircuit decreases the sensitivity of the system. In a like manner,adjustment of the rheostat |08 in a manner to include less of itsresistance in the circuit causes the volta-ge drops across solenoidcoils 45 and 46 to be rebalanced upon a smaller movement of balancingcontact arm 59 along balancing resistance 58. In this manner, a smaller'movement of the instrumentality to be controlled will result upon agiven movement of movable member 56. Likewise, larger movements of theinstrumentality to be controlled will result for a given movement ofmovable member 58 if the rheostat |08 is adjusted in an opposite manner.By means of these two adjustments, the response of the system of thisinvention may be varied as desired or necessary in order to obtain thedesired control. These two adjustments are equally applicable to thesystem even if the other corrective features are omitted.

Rotation of contact finger |60 in a clockwise direction places more ofresistance |6| in circuit with the corrector potentiometer and solenoidcoils |43 and |44 with the result that the movement of plunger |40 willbe decreased for a given movement of movable member |5|.Counterclockwise rotation of contact finger |60 will simcorrectiveadjustment applied to the system in a predetermined time for apredetermined variation from normalmay be varied as desired.

Adjustment of contact fingers |82 and |85 along their respectiveresistances |8| and |84 determines the amount of resistance placed inand out of circuit with the left and right hand portions of controlresistance 55 upon making and breaking of contacts |16 and |18 and |15and |11, respectively, whereby the magnitude of the initial adjustmentgiven to the instrumentality to be controlled upon reverse movements ofmovable member 56 may be varied at will.

While the invention has been specifically described as controlling theflow of fuel to a furnace which it is desired to maintain at a constanttemperature under various load conditions, it will be appreciated thatsuch a system has a great degree of utility in many other fields andthat many modifications in the detailed arrangement illustrated anddescribed may be made without departing from theY spirit of theinvention, wherefore the invention is to be limited only by the scope ofthe appended claims.

I claim as my invention:

l. In combination, an electrically operated device to be positioned in aplurality of positions for controlling the value of a condition,electrical control means for the device including follow-up meansoperated in response to movement of the device and means responsive tothe value of the condition to be controlled i'or graduatinglypositioning said device in accordance with the value of the condition tobe controlled to maintain the condition within certain limits, and meansoperative as an incident to deviation of the condition to be controlledfrom the desired normal value and responsive to the amount of suchdeviation additionally to position the device at a rate corresponding tothe amount of deviation for returning the value of the condition to becontrolled toward the desired normal value.

2. In combination, an' electrically operated device to be positioned ina plurality of positions for controlling the value of a condition,electrical control means for the device including follow-up meansoperated in response to movement of the device and means responsive tothe value of the condition to be controlled for graduatingly positioningsaid device in accordance with the value of the condition to becontrolled to maintain the condition within certain limits, and meansoperative as an incident to deviation of the condition to be controlledfrom the desired normal value and responsive to the duration oi timesuch deviation exists additionally to position the device an amountcorresponding to the duration of time the deviation exists for oreturning the value of the condition to be controlled toward-the desirednormal value.

3. In combination, an electrically operated device to be positioned in aplurality of positions for controlling the value of a condition,electrical control means i'or the device including follow-up meansoperated in response to movement of the device and means responsive tothe value of the condition to be controlled for graduatingly positioningsaid device in accordance with the value of the condition to becontrolled to maintain the condition within certain limits, and meansoperative as an incident to deviation of the condition to be controlledfrom the desired normal value and responsive to the amount of suchdeviation and the duration of time such deviation exists additionally toposition the device an amount and at a rate corresponding to the amountof deviation and the duration of time the deviation exists for returningthe value of the condition toward the desired normal value.

4. In combination, an electrically controlled device to be positioned ina plurality of positions for controlling the value of a condition,control impedance means adjusted in accordance with changes in the valueof the condition to be controlled and having a desired adjustmentcorresponding to a desired normal value of the condition, mechanism,including follow-upl means operated in response to operation of thedevice, controlled by the adjustment of the control impedance means forgraduatingly positioning the device in accordance with the adjustment ofthe control impedance means to maintain the condition within certainlimits, compensating impedance-means for also controlling said mechanismin accordance with the adjustment thereof, and means operative as anincident to deviation of the condition to be controlled from Athe'desired normal value for adjusting .said compensating impedance meansadditionally to position the device for returning the value of thecondition toward the desired normal value.

5. In combination, an electrically controlled device to be positioned ina plurality ot positions for controlling the value of a condition,control impedance means adjusted in accordance with changes in the valueof the condition to be controlled and having a desired adjustmentcorresponding to a desired normal value oi the condition, mechanism,including follow-up means operated in response to operation of thedevice. controlled by the adjustment of the control impedance means forgraduatingly positioning the device in accordance with the adjustment oithe control impedance means to maintain the condition within certainlimits, compensating impedance means for also controlling said mechanismin accordance with the adjustment thereof, and means operative as anincident to deviation of the condition to be controlled from the desirednormal value and responsive to the amount of such deviation foradjusting said compensating impedance means at a rate corresponding tothe amount of deviation additionally to position Vthe device forreturning the value of the condition to be controlled toward the desirednormal value.

6.. In combination, an electrically controlled device to be positionedin a plurality of positions for controlling the value of a condition,control impedance means adjusted in accordance with changes in the valueof the condition to be controlled and having a desired adjustmentcorresponding to `a desired normal value of the condition, mechanism,including iollow-up means operated in response to operation of thedevice, controlled by the adjustment of the control impedance means forgraduatingly positioning the ,device in accordance with the adjustmentoi the control impedance means to maintain the condition "within certainlimits, compensating impedance means for also controlling said mechanismin accordance with the adjustment thereof, and means operative as anincident to deviation of the condition to be controlled from the desirednormal value and responsive to the duration of time such deviationexists for adjusting said compensating impedance means an amountcorresponding to the duration of time the deviation exists additionallyto position the device for returning the value-of the condition to becontrolled toward the desired normal value.

7.1n combination, an electrically controlled device to be positioned ina plurality of positions for controlling the value of a condition,control impedance means adjusted in accordance with changes in the valueof the condition to be controlled and having a desired adjustmentcorresponding to a desired normal value of the condition, mechanism,includingA follow-up means operated in response to operation of thedevice,

controlled by the adjustment of the-control impedance means iorgraduatingly positioning the device in accordance with the adjustment ofthe control impedance means to maintain the Icondition within certainlimits, compensating impedance means for also controlling said mechanismin accordance with the adjustment thereof, and means operative as anincident to deviation oi the condition to" be controlled irom thedesired normal value and responsive to the amount of such deviation andthe duration ot timevsuch deviation existsior adjusting saidcompensating yimpedance means an amount and at a `rate corresponding tothe amount of deviation and/the duration oi time the deviation existsfor returning the value of the condition toward the desired normalvalue.

8. n combination with a condition responsive follow-up control systemfor positioning a device in accordance with changes in the value of thecondition to maintain the condition within certain limits, an automaticresetting mechanism for additionally positioning the device to maintainthe condition at the desired normal value including, resetting means foradditionally positioning the device, a ratchet having a plurality ofratchet teeth for positioning the resetting means, a pawl for engagingthe teeth of the ratchet for operating the same, timing means foroperating the pawl, and means responsive to the amount of deviation ofthe condition from the desired normal value for controlling thepawl toprevent the pawl from engaging any teeth when thev condition is normaland to cause the pawl to pick -up an increasing number of teeth as thecondition deviates from the desired normal value.

9. In combinatiom'adevice to vbe positioned in a plurality of positionsfor' controlling the value of a condition, control means the state ofwhich is varied in accordance with changes in kthe value of thecondition to be controlled and having a desired state corresponding to adesired normal value of the condition, means, including follow-up meansoperated in response to operation of the device, for positioning saiddevice in accordance with the state of the control means to maintain thecondition Within certain limits, and means responsive to changes in thevalue of the condition irrespective of the actual value thereofadditionally to position the device a predetermined amount immediatelyupon change in and means Icontrolled by initial reverse changes in saidcondition regardless of the actual value thereof additionally toposition the device upon change in trend of the value of the condition.

11. In combination, an electrically controlled device to be positionedin a plurality of positions for controlling the value of a condition,control impedance means adjusted in accordance with changes in the valueof the condition to be controlled and having a desired adjustmentcorrespending to a desired normal value of the condition, mechanism,including follow-up means operated in response to operation of thedevice, controlled by the adjustment of the control impedance means forgraduatingly positioning the device in accordance with the adjustment ofthe control impedance means to maintain the condition within .certainlimits, positioning impedance means for also controlling said mechanism,and means responsive to changes in the value of the conditionirrespective of the actual value thereof for controlling the positioningimpedance means additionally to position the device immediately uponchange in trend of the value of the condition. e

12. In combination,` a ,device to be positioned value of a condition,control means the state of which is varied in accordance with changes inthe value of the condition to be controlled and having a desired statecorresponding to a desired normal value of the condition, means,including follow-up means operated in response to operation of thedevice, for positioning said device in accordance with the state of thecontrol means to maintain the condition within certain limits, ,meansoperative as an incident to deviation of the condition to be controlledfrom the desirednormal value additionally to position the device withrespect to the state of the control means for returning the value of thecondition toward the desired normal value, and means responsive tochanges in the value of the condition irrespective of the actual valuethereof additionally to position the device immediately upon change intrend of the value of the condition.

. 13. In a control system, the combination of a controller movablethrough a given range of movement, motor means for performing afunction, follow-up means operated by the motor means, means controlledby the controller and the ,follow-up means for controlling the operationof the motor means to position the motor means in accordance with theposition of the controller w reby the motor means is moved through a gien range of movement upon movement of the controller through a givenrange, and mea associated with the follow-up means for adjng the rangeof movement of the motor me s with respect to the range of movement ofthe controller.

14. In combination, an electrically operated -devicemovable to aplurality of positions within a complete range of movement, controlimpedance means adjustable. through a range of adjustment, balancingimpedance means adjustable through a range of adjustment uponsubstantially complete movement of the device, mechanism controlled bythe-control impedance means and the balancing impedance means toposition the device throughout its range of movement in accordance withthe adjustment of the control impedance means throughout its range ofadjustment, and other impedance means for also controlling the mechanismto adjust the range of movement of the device with respect to the .rangeof adjustment of the control impedancel slider relatively adjustablethrough a range of in a plurality of positions for controlling the 75adjustment, a balancing potentiometer including a resistance element anda slider relatively adjustable through a range of adjustment upon`substantially complete movement of the device, relay means including apair of oppositely acting relay coils for controlling the direction andextent of movement of the electrically controlled device, meansincluding connections between the relay coils andV the ends of theresistance elements of the control and balancing potentiometers forcontrolling the relay means in accordance with the adjustments of thecontrol and balancing potentiometers to position the device throughoutits range of movement in accordance with the adjustment of the controlpotentiometer throughoutrv its range of adjustment, and resistance meansoperatively associated with said balancing potentiometer for alsocontrolling the relaymeans to adjust the range of movement of the devicewith respect to the range of adjustment oi' the control potentiometer.

16. In combination, an electrically operated device movable to aplurality of positions within a complete range of movement, a controlpotentiometer including a resistance element and a slider relativelyadjustable through a range of adjustment, a balancing potentiometerincluding a resistance element and a slider relatively adjustablethrough a range of adjustment upon substantially complete movement ofthe device, relay means including a pair of relay coils connected inseries across a source of power and switching means for controlling thedirection and extent ci movement of the electrically controlled devicedepending upon the relative ener gizations of the relay coils, ilrstconnections between the junction of the relay coils and the sliders ofthe control and balancing potentiometers and second connections betweenthe ends of the resistance elements of the control and balancingpotentiometers and the source of power for controlling the relay meansin accordance with the adjustments of the control and balancingpotentiometers to position the device throughout its range of movementin accordance wtih the adjustment of the control potentiometerthroughout its range of adjustment, and resistance means inv the firstconnections for also controlling the relay means to adjust the range ofmovement of the device with respect to the range of adjustment of thecontrol potentiometer.

l'l. The method of controlling the magnitude of a condition whichcomprises adjusting a control element so that its position at all timessubstantially corresponds to the then existing magnitude of thecondition, producing a control eilect whose sense is determined by thesense of deviation of said element from its neutral position and whosemagnitude is a function of the magnitude of the deviation, producing asecond control eilect whose sense corresponds with and is derived fromthe adjustment of said element, and varying the rate of application ofan agent aiecting the magnitude of said condition in accordance with theresultant of said control eil'ects.

18. 'I'he method of controlling the magnitude of a condition whichcomprises adjusting an element in a sense corresponding to the sense ofthe rate of change oi said condition, deriving fromthe adjusted elementa control eiect in accordance with' said adjustment. producing a secondcontrol eiect in accordance with the devlation of said element from itsneutral position, producing a third control eilect representative of theposition of a member controlling the application of an agent ailectingthe magnitude oi said condition, and adjusting the position of saidmember in accordance with the relative magnitudes of said controleil'ects.

19. 'I'he method of controlling the magnitude of a condition whichcomprises adjusting the position of an element at a rate correspondingto the rate oi' change of the condition so that its deviation from aneutral position at all times substantially corresponds to the existingdeparture o! the magnitude of the condition from the desired magnitudethereof, producing, so long as there is deviation of said element fromits neutral position. a control eiect varying as a function of thedeviation of said element from neutral position, producing a `controleffect in accordance with the rate of adjustment of said element,producing a third control effect in accordance with the position of amember controlling the rate of application of an agent controlling themagnitude of said condition, and changing the position of said member inaccordance with the relative magnitudes of said control effects.

20. The method of controlling the magnitude of a condition whichcomprises positioning a control element in accordance with the existingmagnitude of the condition, varying the position of a second controlelement at a. rate corresponding to the deviation of said condition froma desired magnitude, and varying the rate of application of an agentcontrolling the magnitude of said condition in accordance with therelative position of said elements.

2l, .d control system comprising relatively movable control elementswhose relative positions determine the magnitude of a control effect,means for positioning one of said elements in accordance with themagnitude of a condition, means for adjusting another of said controlelements at a rate related to departure of the magnitude of saidcondition from a predetermined magnitude, a member adjustable to varythe rate of application of an agent controlling the magnitude of saidcondition, means for producing a control effect of magnitude dependentupon the position of said member, and means for adjusting said member inaccordance with the resultant of said effects.

, 22. A system comprising a member defiecting v in accordance with thechange in magnitude of a condition, Aan element so adjusted in responseto deiiections of said member that its position from a neutral positionsubstantially corresponds to the existing deviation of the magnitude ofsaid condition from the desired magnitude, means for varying a controleffect by the adjustment oi' said element, means for varying a controleilect at a rate dependent upon the deviation of said member from itsneutral position, and control means for varying the application of anagent aii'ecting the magnitude of said condition in accordance with theresultant of said control ei- Iects.

23. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition which it is desiredto maintain at a given normal value, a system of control for said deviceincluding control means the state of which is varied in accordance withchanges in the value of said condition and motor means, including followup means operated thereby, to position said device in accordance withthe changes in value of said condition whereby said device has aposition corresponding to each value of said condition throughout apredetermined range of change of said condition, means cooperating withsaid system of control to additionally position said device during atleast a portion of the time that the value of said condition is not atsaid given normal value, and means cooperating with said system ofcontrol for changing the amount oi change o! said condition required tomove said device a predetermined amount.

24. An electrical system of graduate control, comprising, incombination, a control mechanism of the type including a switching meansoperated by an electrical circuit upon unbalanclng of the electricalcircuit, reversible electric motor means controlled by said switchingmeans, a device for varying the value of a condition which it is desiredto maintain at a given value, control means the state of which is variedin accordance with changes in the value of said condition forunbalancing said electrical circuit proportionately to each change invalue of said condition whereby said reversible electric motor means isoperated, means operated by said reversible electric motor means forrebalancing said electrical circuit whereby the device is positioned inaccordance with the variations in said condition to maintain theysaidcondition within a given range but not necessarily at said given value,and manual means for adjusting the unbalancing eiects of said controlmeans upon a predetermined change in said condition in respect to therebalancing effect of said rebalancing means for a given movement ofsaid reversible electric motor means whereby the amount of movementimparted to said device for a given change in the valuevoi" saidcondition may be varied.

25. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition, control meansincludingl a member movable back and forth in response to changes in thevalue of the condition being controlled and having a desired positioncorresponding to a desired normal value of the condition, means,including a variable resistance bridge and follow-up means operated bysaid device, to position said device a predetermined amount in adirection to return said condition to the desired value upon apredetermined movement of said movable member to a new position `awayfrom said desired position and to position said device a differentamount in the opposite direction during the time period comprised by thereaching of said new position by said movable member and its return backto its desired position.

26. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition, control meansincluding a movable member movable back and forth in response to changesin the value of thecondition being controlled and having a desiredposition corresponding to a desired normal value of the condition, acontrol system for positioning said device in accordance with themovement of said movable member and including a variable resistancebridge which is unbalanced upon movementof said movable member and isrebalanced when said device has been moved to a position correspondingto the movements of the movable member whereby said device is adapted tobe returned to the same position each time the movable member returns toits desired position, and means cooperating with said control system andoperative to cause said device to assume a positionvdifferent than itotherwise would upon return of said condition to the desired value.

27.' In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition, control meansincluding a movable member movable back and forth in response to changesin the value of the condition being controlled and having a desiredposition corresponding to a desired normal value of the condition, anormally balanced variable resistance bridge, means to unbalance saidbridge upon movement of said movable member away from its desiredposition and to an extent commensurate with thamovement of said movablemember,.means to rebalance said bridge and simultaneously position saiddevice in a direction to return said condition to the desired value andin an amount commensurate to the unbalance of said bridge .whereby saiddevice is adapted to be returned to its original position upon return ofsaid movable member to its desired position, and means cooperating withsaid resistance bridge and operative to further unbalance said bridge atleast a portion of the time while said movable member is -away from itsdesired position to cause said device to assume a position upon returnof said movable member to its desired position different from that whichit was in when the condition originally varied from the desired value.

28. A control system, comprising, in combination, a resistance bridgeincluding a variable controlling resistance, a variable compensatingresistance and a variable balancing resistance, means responsive to thevalue of a condition which it is desired to maintain constant foroperating said variable controlling'resistance to unbalance said bridge,relay means operated to a rst position upon unbalancing of said bridgein one direction and to a second position upon unbalancing of saidbridge in the opposite direction, means to vary the value of thecondition being controlled, a reversible motor in control thereof andcontrolled by said relay means in a manner to return said condition tothe desired value, means to simultaneously operate said variablebalancing resistance to restore the balance of said bridge, and means tointermittently adjust said variable compensating resistance in a mannerto further unbalance said resistance bridge whenever the value of saidcondition varies from the desired value to cause additional operation ofsaid reversible motor means in a manner to return said condition to thedesired value.

29. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition which it is desiredto maintain at a given normal value, a system of control for said deviceincluding control means the state of which is varied in accordance lwithchanges in the value of said condition, motor means, including followupmeans operated thereby to position said device in accordance with thechanges in value of said condition whereby said device has a positioncorresponding to each value of said condition throughout a predeterminedrange of change of said condition, means cooperating with said system ofcontrol to additionally position said device during at least a portionof the time that the value of said condition is not at said given normalvalue, and means to continuously indicate the summation of theadditional movements imparted to said device by said last named means.

30. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition which it is desiredto maintain at a given normal value, an electric system of control forsaid device including control means to manually adjust the 'amount ofadditional .I

movement imparted to said device by said lastnamed means.

31. An electrical system of graduate control, comprising, incombination, a controlling mechanism of the type including a switchingmeans operated by an electrical circuit upon unbalancing of theelectrical circuit, reversible electric motor means controlled by saidswitching means, a device for varying the value of a condition which itis desired to maintain at a given value, control means the state ofwhich is varied in accordance with changes in the value of saidcondition for unbalancing said electrical circuit proportionately toeach change in value of said condition whereby saidreversible electricmotor means is operated, means operated by said reversible electricmotor means for rebalancing said electrical circuit whereby the deviceis positioned in accordance with the variations in said condition tomaintain the said condition within a predetermined range of change butnot necessarily at said given value, and means cooperating with saidelectrical circuit and operative at least a portion of the time whilesaid condition is at a value other than the desired value to unbalancesaid electrical circuit in a manner to cause said device to assume aposition upon return of the condition to the desired value differentfrom the position in which it waswhen the condition originally variedfrom the desired value, said difference in positions of said devicedepending upon the extent to which the condition varied from the desiredvalue.

32. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition, control meansincluding a movable member movable back and forth in rerate with themovement of said movable meml ber. means to rebalance said bridge andsimultaneously position said device in a direction tov return saidcondition to the desired value and in an amount commensurate to theunbalance of said bridge whereby said device is adapted to be returnedto its original position upon return of said movable member to itsdesired position, and means cooperating with said rebalancing means andoperative at least a portion of the time while said movable member isaway from its desired position to cause said device to assume a positionupon return of said movable member to its desired position differentfrom lthat which it was in when the condition originally varied from thedesired value, said difference in positions of said device dependingupon the extent to which the condition varied from the desired value.

33. A control system, comprising in combination, a resistance bridgeincluding a variable controlling resistance, a variable compensatingresistance and a variable balancing resistance, means responsive to thevalue of a condition which it is desired to maintain constant foroperating said variable controlling resistance to unbalance said bridge,relay means operated to a rst position upon unbalancing of said bridgein one direction and to a second position upon un-h balancing of saidbridge in the opposite direction, means to vary the value of thecondition being controlled, a reversible motor in control thereof andcontrolled by said relay means in a manner to return said condition tothe desired Value, means to simultaneously operate said variablebalancing resistance to restore the balance of said bridge, and means toadjust said variable compensating resistance in a manner to furtherunbalance said resistance bridge an amount dependent upon the length oftime the value of said condition varies from the desired value wheneverthe value of said condition varies from the desired value to causeadditional operation of said reversible motor means in a manner toreturn said condition to the desired value.

34. The method of controlling the magnitude of a condition whichcomprises adjusting a control element so that its position at all timessubstantially corresponds to the then existing magnitude of thecondition,`producing a control effect whose sense is determined by thesense of deviation of said element from its neutral position and whosemagnitude is a function of the magnitude of the deviation and the lengthof time such deviation persists, producing a second control elect whosesense corresponds with and is derived from the adjustment of saidelement, and varying the rate of application of an agent affecting themagnitude of said condition in accordance with the resultant of saidcontrol effects.

35. Aniyelectrical system of graduate control, comp ing, in combination,a controlling mechanism f the type including a switching means operatedby an electrical circuit upon unbalancing of the electrical circuit,reversible electric motor means controlled by said switching means, adevice for varying the value of a condition which it is desired tomaintain at a given value, control meansthe state of which is varied inaccordance with changes in the value of said condition for unbalancingsaid. electrical circuit proportionately to each change in value of saidcondition whereby said reversible electric motor means is operated,means operated by said reversible electric motor means for rebalancingsaid electrical circuit whereby the device is positioned in accordancewith the variations in said condition to maintain the said conditionwithin a predetermined range of change but not necessarily at said givenvalue, means for additionally unbalancing said electrical circuit upondeparture of the value of Said condition from said given value and in anamount dependent upon the amount of said departure and the length oftime said departure persists whereby said device is additionallyoperated by said reversible electric motor means as the latter operatessaid rebalancing means to rebalance said electrical circuit.

36. In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition, control means thestate of which is varied in accordance with changes in the value of thecondition to be controlled and having a desired state corresponding to adesired normal value of the condition, means, including followup meansoperated in response to operation ci' the device, for positioning saiddevice in accordance with the state of the control means to maintain thecondition within certain limits, means responsive to changes in thevalue of the condition irrespective of the actual value thereofadditionally to position the device in a direction to reverse the trendin the change of said condition immediately upon change in trend ci? thevalue of the condition, and means operated as an incident to deviationoi the condition from the desired value 'and responsive to the amount ofsuch deviation additionally to positionthe device at a ratecorresponding to the amount of such deviation for returning the value ofthe condition to be controlled toward the desired normal value.

3'1` In combination, a device to be positioned in a plurality ofpositions for controlling the value of a condition, control means thestate of which is varied in accordance with changes in the rvalue of thecondition to be controlled and having a desired state corresponding to adesired normal value of the condition, means, including ,follow-up meansoperated in response to operation of the device, for positioning saiddevice in accordance with the state of the control means to maintain thecondition within certain limits, means responsive to changes in thevalue of the condition irrespective of th:y actual value thereofadditionally Vto position the device in a direction to reverse the trendIin the change ofvsaid condition immediately upon change in trend of thevalue of the condition, and means operated as an incident to deviationof the condition from the desired value and responsive to the durationof time such deviation exists additionally to position the device anamount corresponding to the duration of time 'the deviation exists forreturnins the condition to be controlled toward the desired normalvalue.

38. An electrical system of graduate control,

comprisinmin combination, a controlling mechanism of the type includinga switching means operated by an electrical circuit upon unbalancing ofthe electrical circuit, reversible electric motor means controlled bysaid switching means to rotate in opposite directions upon unbalance insaid circuit in opposite directions, a device for varying the value of acondition which it is desired to maintain at a given value, controlmeans the state of which is varied in accordance with changes in thevalue of said condition for un-. i

balancing said electrical circuit proportionately to each change invalue of said condition whereby said reversible electric motor means isoperated. means operated by said reversible electric motor means forrebalancing said electrical circuit wherebyv the device is positioned inaccordance with the variations in said condition tomaintain 'thesaidcondition "within a" predetermined range

